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Applying a membrane tank concept for large container ships

Large ship types are the next frontier for LNG propulsion technology, with proven fuel tank concepts already being adapted for the long-range LNG operation of container vessels.

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The advent of LNG as a low-emission, comparatively cost-efficient future fuel for merchant ships has prompted a surge of development activities across the shipbuilding industry. Hanjin, GTT and DNV GL have signed an agreement to jointly investigate and develop a gas-fuelled large container vessel concept equipped with membrane fuel tanks.

The study focuses on a 16,300 TEU container ship equipped with two membrane tanks capable of bunkering up to 11,000 cubic metres, enough for around 15,000 nautical miles. The ship is intended to travel between Asia and Europe and will have to cross at least one Emission Control Area (ECA) in European waters, where strict sulphur emission limits are now in effect.

Hanjin is designing the key components for the LNG supply system, GTT is responsible for the integration of the fuel containment system, and DNV GL is handling the design review, hazard identification and, upon successful completion of the project, the Approval in Principle (AiP) of the design. The safety performance assessment for the gas supply system and the tank system integration are key aspects of DNV GL’s contribution.

Efficient LNG storage

The project focused on the LNG fuel system, consisting of the bunker station, LNG fuel tanks, gas preparation and fuel supply systems. Apart from the technical aspects, the project also investigates economic feasibility criteria. This includes evaluation of the LNG tank location and range in gas mode based on the ship’s operational profile, outlining the requirements for an LNG-fuelled design, and an assessment of prospective LNG availability at relevant locations.

The biggest advantage of membrane tanks is that they make efficient use of the space available on board. The Mark III membrane system chosen for this large container vessel concept consists of a cryogenic liner directly supported by the ship’s inner hull. The liner is composed of a primary metallic membrane with an insulation layer and a secondary membrane underneath.

The boil-off challenge solved

One key issue has been the treatment of boil-oil gas from the LNG tanks. The pressure increase inside the membrane tank system must be limited, without releasing gas to the atmosphere. This can be achieved by using the boil-off gas to power the auxiliary engines and the boiler.

Most of the time, the power demand by far exceeds the natural boil-off from the tanks, so the system must actually vaporise additional volumes of LNG to meet the fuel demand, while maintaining a low operating pressure inside the tanks. However, when the ship is idle and the power demand is very low, gas pressure will build up inside the tank. As long as some gas is drawn to power the minimum hotel load, the pressure will increase relatively slowly.